1. Field of the Invention
The present invention relates to an image reading apparatus, such as a stand-alone scanner apparatus or a scanner apparatus installed in a copier or facsimile apparatus, and to an image forming apparatus, such as a copier or multifunction peripheral, that includes an image reading apparatus, and, more particularly, to an image reading apparatus that reads a document image by using reading units that are divided in the main scanning direction and to an image forming apparatus.
2. Description of the Related Art
An image reading apparatus, such as a stand-alone scanner apparatus or a scanner apparatus installed in a copier or facsimile apparatus, includes a conveyance drive roller 1a and a conveyance follower roller 1b that convey a document, which is placed on a document table 8 and inserted in an insertion direction, as illustrated in FIG. 16; a conveyance drive roller 2a and a conveyance follower roller 2b; and a read sensor 3 (including a light source 5, a Selfoc lens 6, and a light receiving element 7, such as a charge-coupled device) that reads an image of a document from below a contact glass 4.
The conveyance drive roller 1a, the conveyance follower roller 1b, the conveyance drive roller 2a, and the conveyance follower roller 2b each have a rubber layer formed on the outer circumference thereof. Because the rubber layer contracts or expands due to a change in the surrounding temperature, the outer diameter of each of the conveyance drive roller 1a, the conveyance follower roller 1b, the conveyance drive roller 2a, and the conveyance follower roller 2b is changed when there is a change in the temperature.
As illustrated in FIG. 17A, the length of the read sensor 3 in the longitudinal direction (the document width direction) is designed to cover the maximum width of a document 9 to be read. The provision of the read sensor 3 allows acquisition of high-quality images with a simple configuration and image processing without being affected by a change in the outer diameters of the conveyance drive roller 1a, the conveyance follower roller 1b, the conveyance drive roller 2a, and the conveyance follower roller 2b. 
The cost of components of a read sensor increases exponentially in relation to the length of the read sensor in the width direction. If a reading unit includes a single read sensor 3, as illustrated in FIG. 17A, the cost of the image reading apparatus is increased.
As illustrated in FIG. 17B, in some configurations, three read sensors 3a and 3b, each having a smaller width than the maximum width of the document 9, are arranged in two rows in, what is called, a staggered manner (for example, see Japanese Patent Application Laid-open No. S59-105762).
If the image reading apparatus illustrated in FIG. 16 uses the read sensors 3a, 3b illustrated in FIG. 17B, the image reading apparatus needs to include an image processing unit for combining sets of digital image data that are obtained by performing analog processing, analog-digital conversion processing, and the like on sets of analog image data acquired by the read sensors 3b and 3b at different timings. In order to connect the images correctly, it is necessary to ensure the positional accuracy of the read sensors 3a and 3b. The positional accuracy of the read sensors 3a and 3b is ensured by ensuring the accuracy of components and by adjusting the installation of components.
In a conventional image reading apparatus, such as that described above, the outer diameters of the conveyance drive rollers 1a and 2a, which convey a document to a read position, change due to changes in the temperature, and the speed for conveying a document is also changed in accordance with a change in the outer diameters of the conveyance drive rollers 1a and 2a; therefore, if the read sensors 3a and 3b, each having a smaller width than the maximum width of a document, are arranged in a staggered manner, image misalignment in the conveying direction occurs at the connection area of the images read by the read sensors 3a and 3b. 
FIGS. 18A and 18B illustrate an example of the above-described image misalignment. An image 9a, as illustrated (with diagonal lines) in FIG. 18A, is not affected by a change in the outer diameters of the conveyance drive rollers 1a and 2a and is obtained by correctly combining sets of image data acquired by a plurality of read sensors.
If the document conveying speed is changed in accordance with a change in the outer diameters of the conveyance drive rollers 1a, 2a, although the image processing unit combines the images taking into consideration the delay time T due to the distance L between the read sensors 3a and 3b in the conveying direction, image misalignment in the conveying direction (the sub-scanning direction) occurs at the image connection area (indicated by a dashed line in the drawing) on an image 9b, as illustrated (with diagonal lines) in FIG. 18B.
Japanese Patent No. 3756474 discloses a technique that prevents the occurrence of image misalignment in the conveying direction at an image connection area if the document conveying speed is changed in accordance with a change in the outer diameters of the conveyance drive rollers 1a and 2a as described above.
Japanese Patent No. 3756474 discloses an image reading apparatus that includes a first sensor that reads a part of a document that is divided in the main scanning direction; a second sensor that is located downstream in the sub-scanning direction of the first sensor and that reads a different part of the document other than the part of the document; a conveyance drive roller that conveys a document to the first and second sensors; a core bar to which the conveyance drive roller is attached such that the core bar rotates the conveyance drive roller; a delay unit that delays an output signal from the first sensor in accordance with the set interval between the first sensor and the second sensor in the sub-scanning direction; an image combining unit that combines the first sensor output signal delayed by the delay unit with an output signal from the second sensor so as to obtain image signals; and an adjusting unit that adjusts the interval between the first sensor and the second sensor in accordance with a change in the conveying speed of the conveyance drive roller.
In this image reading apparatus, the interval (the set interval) between the first sensor and the second sensor is changed in accordance with any change in the conveying speed due to thermal expansion and thermal contraction of the conveyance drive roller; therefore, while the delay time pre-set in the delay unit is used, the occurrence of image misalignment in the conveying direction is avoided when a partial image (a part of an image) read by the first sensor is combined with a partial image (a different part of the image other than the part of the image) read by the second sensor.
Such a conventional image reading apparatus does not take a measure to prevent warping of the core bar when the conveyance drive rollers are fixed to the core bar.
Specifically, if a plurality of conveyance drive rollers is attached to a rotary shaft such that the outer circumference surfaces of the conveyance drive rollers are in contact with a document, a small-diameter rotary portion is formed on the conveyance drive roller, extending in the main scanning direction, and a through-hole, through which a screw is inserted, is formed on the small-diameter rotary portion.
A screw hole, with which a screw is engaged, is formed on one side of a rotary shaft in a direction perpendicular to the axis direction of the rotary shaft. The screw holes are formed on the rotary shaft with a space interposed therebetween in the axis direction of the rotary shaft.
To fix a conveyance drive roller to the rotary shaft, after the conveyance drive roller is attached to the rotary shaft, a screw is inserted into the through-hole on one side of the rotary shaft and engaged with the screw hole of the rotary shaft.
The conveyance drive roller is fixed to the rotary shaft, as described above, so that it is possible to prevent the head portion of the screw from protruding outward from the outer circumference surface of the conveyance drive roller such that the head portion of the screw is not brought into contact with a document.
However, when the conveyance drive rollers are fixed to the rotary shaft, the screws are attached to one side of the rotary shaft, which results in the application of fastening forces only to one side of the rotary shaft.
Therefore, the rotary shaft warps in a direction perpendicular to the rotation center of the rotary shaft, and during rotation of the rotary shaft, the rotary shaft is moved in a direction perpendicular to the rotation center of the rotary shaft. Thus, a change in the conveying speed of the conveyance drive roller due to thermal expansion and thermal contraction becomes significant because of the movement of the rotary shaft in a direction perpendicular to the rotation center, and therefore there is a possibility that it is not possible to avoid the occurrence of image misalignment in the conveying direction at the image connection area.
Particularly, when a document with a large reading width is read, it is necessary to attach a large number of conveyance drive rollers to a longer rotary shaft; therefore, in the case of a longer rotary shaft, warping of the rotary shaft becomes more significant.